In general, in textile dyeing of cotton, vat dyes have been proven to have excellent fastness properties over other dyes. Reactive dyes are the next to the vat in the rating. To have an efficient dyeing operation, dyer has a responsibility of controlling his system variables such as temperature, time, liquor ratio, dye concentration, pH, chemicals, salt and so on.
Aim: To evaluate dyeing variables such as liquor ratio, dyeing time and dye concentration at a constant range of temperature in order to determine economical and optimum application of reactive and the vat dyes on mercerized 100% cotton fabric.
The sample was subjected to preparatory processes: desizing, scouring, bleaching and mercerization. Dyeing of the sample took place in 6-cylindrical dyeing machine in which the baths were immersed in water inside a trough. The samples from the mercerized cotton fabric were subjected to dyeing at different conditions.
|Variable||Liquor ratio||Dyeing time (min)||Concentration of dye (% o.w.f)||Temp (◦C)|
Method of analysis: Random visual assessment by four different judges in which the samples were rated according to intensity or value and three out of the observers followed similar rating independently. This caters for differences in an individual’s power of vision.
Liquor ratio: The intensity of the reactive shown significant difference at different liquor ratios and the results increased in descending order (30:1, 40:1, 50:1) while vat dye showed a difference in ascending order (50:1, 40:1, 30:1). The liquor ratio of 40:1 is an intercept which favored both dyes.
Dyeing time/period: There were significant differences at a different time. Reactive dye showed the highest intensity at first 30 minutes and the least at 45 minutes. Vat dye increased in value at 60 minutes and least at 75 minutes.
Concentration of dye: At depth of shade of 4%, 8%, and 12% respectively, the intensity of both reactive and the vat dyes is proportional to the concentration of dye and better brilliance was achieved by the reactive dye.
Keywords: mercerization, reactive dye, vat dye, color intensity
Introduction/Background to the study
Textile dyeing is an application of colorant; usually, dye to the textile material in an aqueous solution or medium of dye and other additives. Effective and efficient dyeing requires appropriate monitoring of system variables which include: temperature, time, liquor ratio, dye concentration, pH, and chemicals. All these variables exercise considerable influence on dyeing in terms of exhaustion and color intensity of dyed sample.
Methods (experimental) and materials
The sample was subjected to preparatory processes which consist of desizing, scouring, bleaching and mercerization before being dyed at a restricted temperature of 50 -600C for seven different baths as in the table-1. Non-parametric statistics techniques were adopted in an analysis of the dyed samples and ordinal scale rating was used to express a relative degree of intensity of the dyed samples, strongest = 5, stronger = 3 and strong = 1.
Mercerized 100% cotton (plain weave fabric), reactive dye (Cibacron yellow F-4G), common salt 50g per litre of bath, sodium carbonate 10g per litre of bath,6-cylindrical dyeing machine, beakers, stirrer, measuring cylinder, weighing balance, thermometer, stop wash, anthraquinone vat dye (yellow), sodium hydrosulphite (5% o.w.f), caustic soda (20% o.w.f), common salt (10% o.w.f).
Reactive dye was weighed, pasted and mixed at room temperature with water and added to the dye bath. Subsequently, the mercerized samples were immersed and agitated for 5 minutes in different baths. Later, the samples were removed and 50g/l of common salt was added to each bath separately. The samples were later reintroduced into the baths and the temperature was raised to boiling within a period of 30 minutes. Thereafter, the samples were removed from the baths and 10g of sodium carbonate was added to each bath respectively and dyeing continued at a temperature range of 50-600C. For vat dye, it was pasted and dissolved in hot water about 50ml. The dissolved caustic soda and the sodium hydrosulfite were added to each bath separately. Moist mercerized cotton samples were immersed in the dye liquor and dyeing continued with agitation at temperature ranges of 50-600C. At the completion of dyeing, samples were hung in the air for 10-30 minutes to allow dye air oxidation. Subsequently, the samples were rinsed in diluted acetic acid (concentration: 0.1%) before hot and the cold water rinse.
|Reactive dye||Reactive dye||Reactive dye||Vat dye||Vat dye||Vat dye|
|Baths||Liquor ratio||Dyeing time (min)||Concentration of dye (% shade)||Liquor ratio||Dyeing time(min)||Concentration of dye (% shade)|
Table 1: variables and their values with reactive and the vat dyes in different dyeing baths.
Results / findings
|Baths /s/n||Liquor ratio||Dyeing time (min)||Concentration of dye (%shade)||Corresponding color intensity rating (ordinal scale)|
Table 2: Reactive dye ( Cibacron F-4G ) yellow, variables & the color intensity rating
|Baths / s/n||Liquor ratio||Dyeing time (min)||Concentration of dye ( % shade)||Corresponding color intensity rating (ordinal scale)|
Table 3: Vat dye (anthraquinone) yellow, variables & color intensity rating.
Discussion and conclusion
From the graphs (1& 4) of dyeing with Cibacron Yellow F-4G and the vat; for reactive, color intensity is high at short liquor ratios while the reverse is the case with the vat dye. This suggests that reactive dyeing is ideal with dyeing equipment using short liquor ratio such as Jigger, while vat dyeing is apt with long liquor ratio equipment. Part of the advantages of reactive dyeing is, therefore, energy cost, water, dye, and additives economy as well as low effluent discharge.
Vat dyeing, on the other hand, has the advantage of uniform dyeing. The graphs (2&5) time of dyeing against color intensity for both dye shows a non-linear relationship. Reactive dye has the highest color intensity at first 30 minutes and maximum curve turning point at 60 minutes which corresponds to color intensity (3). This suggests that covalent bonds between dye molecules and fiber are more reactive at early dyeing time before optimum exhaustion rather than prolonged time. For the vat dye, however, the intensity shows the highest value (5) at 60 minutes before finally dropping to one (1) as dyeing continued; the minimum turning point of the curve corresponds to 45 minutes of dyeing. This reverse in the process occurred because oxidation, the fixation means for vat dyes, had not taken place immediately after the optimum exhaustion. This indicates that there is a time limit for the application of vat dyes, beyond which the color drifts back to the solution.
In the case of dye concentration, the graph (3) shows that both reactive and the vat dyes have a linear relationship between the concentration of applied dye and color intensity before equilibrium exhaustion. Finally, the graphs generally indicate a higher sensitivity of color intensity to liquor ratio and time of dyeing than it is to concentration; viewed from the stiffness of the various trend lines.
It can be concluded from the above results that dye concentration, liquor ratio and time of dyeing have direct effect on the intensity of their resulting colors. While reactive dyeing is exhausted better at short liquor ratio; 30:1 or below, vat dyeing is more ideal at long liquor ratio (50:1). For both the reactive and vat dyes, the intensity has a direct relationship with dye concentration. The two dyes, however, differ in the manner of relationship between the intensity of their colors and time of dyeing. For the reactive dye, the longer the time of dyeing the higher the intensity but for the vat dye, the relationship was initially direct but as the time of dyeing went on, the intensity declined. Thus, vat dyes should be applied to cotton for a maximum period of sixty (60) minutes.
Lastly, the intensity of the color is more sensitive to liquor ratio and time of dyeing than it is to dye concentration. In term of the concentration of dye, the theory of isotherm is applicable to both reactive and the vat dyes respectively.
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